The invention relates to a permanent magnet excited electric machine, comprising:
a rotor having one or more permanent magnets;
a stator having one or more coil windings; and
an electronic switching means for coupling current into the coils and/or for coupling out current from the coils.
Such machines are generally known. Usually, the rotor is the exciting part carrying permanent magnets, which is arranged in a predetermined position so as to be rotatable in relation to the stator. With the aid of the electronic switching means, energy is coupled into the coils, for example, from a direct voltage intermediate circuit so as to operate the machine as a motor. In the generator-mode of operation, the electrical energy generated in the individual coil windings by rotation of the rotor is coupled out via the electric switching means.
According to Faraday""s law of induction, a voltage is induced in the individual coil turns, and the sum of the individual turn voltages is present at the terminals of the coil windings.
If, with the machine running, a short circuit occurs in a turn or in or across several turns of a stator winding, the magnetic alternating flux flowing through the coil windings causes a short-circuit current in the shorted turn or turns.
In accordance with Lenz""s law, the short-circuit current has a strength according to which the permanent magnetic alternating flux is eliminated. The magnetic alternating flux induced by the short-circuit current thus has the same value as the alternating flux caused by the permanent magnets and is of opposite phase in relation to the latter.
The short-circuit currents flowing in one or more or across several short-circuited turns are in excess of the nominal coil currents by a multiple, and there are thus arising considerable thermal losses. As a rule, the stator windings then cannot be cooled sufficiently, thus causing burning off of the particular turn or turn portion in the stator winding.
It is the object of the invention to develop a permanent magnet excited electric machine of the type mentioned in such a manner that, in case of an interturn short-circuit, destruction of the turn or turn section affected by the short-circuit can be prevented.
In addition thereto, a method of operating a permanent magnet excited electric machine is to be indicated through which, in the event of an interturn short-circuit, destruction of the shorted turn is avoided.
To meet the object indicated hereinbefore, the electric machine according to the invention is provided with
a short-circuit detection means for detecting a short-circuit in one or more turns of a coil, and
a compensation means for effecting a compensation current flow in at least that coil that contains the short-circuited turn.
In a method of operating a permanent magnet excited electric machine having the features indicated at the outset, the invention provides for the following steps:
monitoring each coil so as to detect a possible interturn short-circuit in the coil, and
in the event of a short-circuit, effecting a compensation current flow in at least that coil that contains the short-circuited turn.
The invention is based on the finding that, in the event of an interturn short-circuit, it is indeed not possible to prevent the generation of a short-circuit current resulting in an electrically induced magnetic alternating flux corresponding in value to the permanent magnetic alternating flux, but that this short-circuit current may be distributed also to such turns that are not affected by the short-circuit, but are magnetically coupled with the turn affected by the short-circuit.
In an embodiment of the invention, upon detection of a short-circuit in a turn or a turn section of a coil winding, the coil concerned is short-circuited at the terminals thereof, so that a short-circuit current arises in the entire coil that results in an electrically induced magnetic alternating flux corresponding in its value to the permanent magnetic alternating flux and being of opposite phase relative to the same. Due to this process, the turn affected by the short-circuit is relieved. The short-circuit current of the affected turn is reduced considerably, so that the losses caused by the xe2x80x9cdistributedxe2x80x9d short-circuit current can be compensated by the coolants provided so as to thus prevent burn-off of the affected turn.
Especially with electric machines having a stator composed of individual coils that are linked to each other in weakly magnetic manner only, the short-circuit current flowing through the entire coil corresponds approximately to the maximum operating current flowing through the coil winding during normal operation. Thus, the cooling provided for the normal operating current is sufficient also in the event of an interturn short-circuit for compensating the thermal losses due to the short-circuit current flowing in the affected turn as well as the short-circuit current flowing in the remainder of the coil winding.
In the event of a short-circuit, the machine can be brought to a standstill either rapidly and in uncontrolled manner or slowly and in controlled mannerxe2x80x94depending on the particular application. In case of an electric machine with individual coils, it is possible in the event of an interturn short-circuit that the affected coil is short-circuited, but the remaining coils can be controlled as in normal operation, i.e. controlled deceleration of the motor can be achieved. However, it is also possible that one or more additional coils are short-circuited, up to the extreme case that all coils are short-circuited. In that event, extremely rapid stopping of the machine takes place which, however, is indeed tolerable in some cases.
The initiation of a compensation current flow, i.e. of a short-circuit current in the part of one or more coils not affected by the interturn short-circuit, can be realized in terms of circuit technology preferably by a switching means between the terminals of the coil winding. With the aid of a relay or a contactor, the short-circuit of the stator winding containing the turn part affected by the short-circuit then takes place. In this respect, it is particularly advantageous that the then flowing short-circuit current automatically has the correct phase with respect to the short-circuit current in the affected turn.
As an alternative to the switching means between the terminals of the coil winding or also as a supplementation of such a switching means, the compensation means may also be constituted by the electronic switching means that is present anyway and controls coupling in and coupling out of the currents into the coil winding and from the coil winding, respectively. The electric machine usually comprisesxe2x80x94both as motor and as generatorxe2x80x94an electronic control means that feeds the coil winding from a direct voltage intermediate circuit or couples energy from the coil winding into a direct voltage intermediate circuit at the appropriate time. In the event of a short-circuit, this electronic switching or control means then is set in particular to continuous current passage. The current flow in the electronic control means then corresponds approximately to the maximum nominal operating current in normal operation of the machine. If there is provided a separate switching means, e.g. a relay or contactor, for short-circuiting the stator, the control means is opened after closing of this switch for short-circuiting the stator winding.
If the compensation current flow is effected or initiated with the aid of the electronic control means, the latter may feed actively controlled current into the coil inflicted with the short-circuit, with the current direction then being set such that the current flows in-phase with respect to the short-circuit in the affected winding. The electronic switching or control means not belonging to the short-circuited coils, or also part of these means, then operate as if the machine were operating in the normal mode.
As mentioned hereinbefore, it is possible specifically with an electric machine with individual coils that one, several or all individual coils are short-circuited or have a compensation current applied thereto in case a coil is affected by an interturn short-circuit. The application of a compensation current to all coils results in deceleration of the machine with the highest possible braking moment. This may have to be tolerated or even be desired in some cases, but may lead to dangerous situations in other cases, for example when the electric machine is used as a drive for a vehicle. In that case, it is to be preferred to act only on the coil affected by the interturn short-circuitxe2x80x94and, if applicable, on one or several additional coilsxe2x80x94whereas the remaining coils of the machine are controlled so as to achieve controlled deceleration of the machine.
While the above statements relating to the invention and specific and preferred embodiments of the invention are concerned with the generation of compensation current flows in the event of an interturn short-circuit, the recognition or detection of an interturn short-circuit shall be discussed in more detail in the following. As short-circuit detection means, it is possible to provide e.g. a temperature sensor for each coil winding. Due to the fact that an interturn short-circuit leads to a short-circuit current with correspondingly rapid generation of dissipated heat, this dissipated heat can be utilized as indication for an interturn short-circuit. This measure in particular has the advantage that temperature sensors possibly may be provided anyway for the normal operation of the machine, i.e. that already existing temperature sensors may be used for recognizing or detecting interturn short-circuits. The output signals of the temperature sensors are detected by a short-circuit detector/compensation control and converted to control signals for causing short-circuit currents, e.g. by closing the afore-mentioned switches between the terminals of the coil windings or by through-control of the electronic switching valves belonging to the affected coil winding.
In a modified embodiment, the short-circuit detection means comprises an inductance measuring means for detecting the respective coil inductance. In the event of an interturn short-circuit, the current curve or flow in the connecting lines connected to the coil winding changes. With the aid of current sensors coupled to these lines, the current in the lines can be detected. By differentiating the current flow and comparing the derivative value for the current thus obtained with a threshold value, it is possible to ascertain current curve edges that have become steeper. Extremely steep edges in the current curve indicate a reduction of the coil inductancexe2x80x94which is due to an interturn short-circuit. The evaluation of the coil currents for detecting inductance losses in the individual coils may take place in a central short-circuit detector/compensation control, but preferably evaluation and short-circuit generation take place directly in the electronic control means of the coil concerned.
As already mentioned hereinbefore, particularly simple and uncomplicated compensation in case of interturn short-circuits can be achieved when the stator is composed of individual coils. The outputs of the coils may be electrically linked together and may be operated correspondingly by the electronic switching means. However, it is particularly preferred to use the invention in case of such machines in which the stator is composed of mutually independent individual coils. Each coil has a single-phase inverter separately associated therewith. The detection of short-circuits and initiation of compensation currents take place separately in the branch of the affected coil. This can take place either in completely independent manner of the remaining parts of the stator, or it is possible to provide for coordinated measures with the aid of a central means.